Respiratory disease has a major impact on children’s health, and objective measures of lung function are critical for the care and study of pediatric lung disease. Understanding lung development provides a conceptual framework for pulmonary function test (PFT) findings in children. Although the conventional age at which spirometry can be performed in children is 6 y, many children under this age can successfully perform spirometry. Other pediatric PFTs include oscillometry, multiple breath washout, measurement of specific airway resistance, and the interrupter technique. PFTs can also be performed in infants, but it requires a high degree of expertise and specialized equipment.